What are you talking about? What laws of physics say an object, artificial or otherwise, can't travel from one solar system to another? Heck, whatever Oumuamua is, it's an existence proof to the contrary.
Should I clarify that when we talk about the spaceship hypothesis, we're not saying we think it's an *operational* spaceship full of living aliens? A multimillion-year-old, and (duh) long-dead, spaceprobe would fit the bill too.
Is there some law of physics I'm not aware of that says it's impossible that, millions of years from now, our Pioneer 11 (for example) might drift close enough to some other star to make a similar hyperbolic swing around it and cause the aliens living there to have this same argument?
(Of course I'm not saying it's likely, or that there's any scientific evidence for it, but why should it be *impossible*?)